The present invention relates to an ignition system of an internal combustion engine.
In gasoline engines, it is possible to improve the specific fuel consumption and reduce the amount of harmful NO.sub.x and CO components in the exhaust gas by increasing the air-fuel ratio of the fuel mixture fed into the cylinders of the engine, that is, by using a lean air-fuel mixture. Therefore, at present, the use of such a lean air-fuel mixture to cause combustion is considered one of the most preferable combustion methods. However, a lean air-fuel mixture is inherently less easy to ignite. Even if the lean air-fuel mixture is ignited, since the propagating speed of flame of the lean air-fuel mixture is low, the burning velocity of the lean air-fuel mixture is low. Therefore, when such a lean air-fuel mixture is used, problems occur in that it is difficult to obtain good combustion, in that the specific fuel consumption will deteriorate, and in that the amount of unburned hydrocarbons in the exhaust gas will be increased. Methods to eliminate such problems by increasing the ignition energy or increasing the size of the spark gap of the spark plugs are known. However, while the use of such methods improves the ignition, it makes it impossible to increase the burning velocity of a lean air-fuel mixture. A method to increase the burning velocity of a lean air-fuel mixture, by causing a strong turbulence of the lean air-fuel mixture is known. However, too strong a turbulence of the lean air-fuel mixture extinguishes, the flame of the lean air-fuel mixture ignited by the spark plug, thus, the turbulence of the lean air-fuel mixture cannot be made too strong.
An engine capable of increasing the burning velocity of an air-fuel mixture is disclosed in Japanese Utility Model Laid-Open Publication 49-42322. In this engine, a plurality of spaced electrode pieces is arranged aligned with each other in series for forming spark gaps between adjacent electrode pieces. A high voltage is applied across the electrode pieces so that a discharge arc is created in the plurality of spark gaps formed between adjacent electrode pieces arranged in series. In this engine, even if the propagating speed is the same as that in a conventional engine, since the combustion is started from a plurality of points in the combustion chamber at the same time, the length of time during which the combustion is carried out is shortened. This makes it possible to increase the burning velocity of an air-fuel mixture. However, since this engine is constructed with each of the electrode pieces merely formed by a bar member and with a discharge arc created in a plurality of the spark gaps arranged in series, a problem occurs in that increased voltage is necessary across the electrode pieces for creating a discharge arc.